Subir Mukhopadhyay

Phase Transitions in Higher Derivative Gravity

This paper deals with black holes, bubbles and orbifolds in Gauss–Bonnet theory in five dimensional anti de Sitter space. In particular, we study stable, unstable and metastable phases of black holes from thermodynamical perspective. By comparing bubble and orbifold geometries, we analyse associated instabilities. Assuming AdS/CFT correspondence, we discuss the effects of this higher derivative bulk coupling on a specific matrix model near the critical points of the boundary gauge theory at finite temperature. Finally, we propose another phenomenological model on the boundary which mimics various phases of the bulk space-time.

Phase transitions in higher derivative gravity and gauge theory: R-charged black holes

This is a continuation of our earlier work where we constructed a phenomenologically motivated effective action of the boundary gauge theory at finite temperature and finite gauge coupling on S3 × S1. In this paper, we argue that this effective action qualitatively reproduces the gauge theory representing various bulk phases of R-charged black hole with Gauss-Bonnet correction. We analyze the system both in canonical and grand canonical ensemble.

Conifolds from D-branes

Abstract In this note we study the resolution of conifold singularity by D-branes by considering compactification of D-branes on C 3 /( Z 2 × Z 2 ) . The resulting vacuum moduli space of D-branes is a toric variety which turns out to be a resolved conifold, that is a nodal variety in C 4 . This has the implication that all the corresponding phases of Type–II string theory are geometrical and are accessible to the D-branes, since they are related by flops.

On 5D small black holes

Using higher order corrections we argue that five dimensional = 2 and = 4 small black holes exhibit supersymmetry enhancement in near horizon geometry leading to eight and sixteen supercharges, respectively. Using this enhancement at supergravity level we can identify the global supergroup of the near horizon geometry. In particular we show how this supergroup distinguishes between small and large black holes in = 2 case.

Fractional branes in non-compact type IIA orientifolds

We study fractional D-branes in the Type-IIA theory on a non-compact orientifold of the orbifold 3/3 in the boundary state formalism. We find that the fractional D0-branes of the orbifold theory become unstable due to the presence of a tachyon, while there is a stable D-instanton whose tachyon gets projected out. We propose that the D-instanton is obtained after tachyon condensation. We evidence this by calculating the Whitehead group of the Abelian category of objects corresponding to the boundary states as being isomorphic to 2.

String network and U duality

We discuss the generalization of recently discovered BPS configurations, corresponding to the planar string networks, to non-planar ones by considering the U-duality symmetry of type II string theory in various dimensions. As an explicit example, we analyze the string solutions in 8-dimensional space-time, carrying SL(3) charges, and show that by aligning the strings along various directions appropriately, one can obtain a string network which preserves 1/8 supersymmetry.

ON SIX-DIMENSIONAL FUNDAMENTAL SUPERSTRINGS AS HOLOGRAMS

In this paper we discuss a possible holographic dual of the two-dimensional conformal field theory associated with the world-sheet of a macroscopic superstring in a compactification on a four-torus. We assume that the near-horizon geometry of the black string has symmetries of AdS3×S3×T4 and construct a sigma model in the bulk. Analyzing the symmetries of the bulk theory and comparing them with those of the CFT in a special light-cone gauge, we find agreement between global symmetries. Due to nonstandard gauge realization it is not clear how affine symmetries can be realized.

D-BRANE INTERACTION IN THE TYPE IIB MATRIX MODEL

Abstract We calculate the potential between bound states of D-branes of different dimension in IIB matrix model upto one loop order and find nice agreement with the open string calculations in short and large distance limit. We also consider the scattering of bound states of D-branes, calculate the scattering phase shift and analyze the effective potential in different limits.

U DUALITY AND NETWORK CONFIGURATIONS OF BRANES

We explicitly write down the invariant supersymmetry conditions for branes with generic values of moduli and U duality charges in various space–time dimensions D≤10. We then use these results to obtain new BPS states, corresponding to the network type structure of such branes.

String multiplets from an invariant 2-brane

We study a three-dimensional gauge theory obtained from the dimensional reduction of a D4-brane worldvolume theory in the background of space-time moduli. An SL(3) symmetry in this theory, which acts on fields as well as coupling constants, is identified. By comparing the energies with the string tensions, we show that certain 1/2 supersymmetric classical solutions of this theory can be identified as SL(3,Z) multiplets of type II strings in eight dimensions. Results are then generalized to the non-linear Born-Infeld action. We also discuss the possibility of 1/8 BPS states in this theory and their representations in terms of string networks.